Studies are proposed which aim to gain further information that is relevant to organ transplantation. A highly reproducible, easily quantitated in vivo model which employs surgical transplantation of thyroid lobes under the kidney capsule of different recipients will be used to evaluate the in vivo relevance of in vitro findings. Protocols relating to the strong transplantation antigens that evoke the cell mediated immune response that leads to rejection of allogenic grafts and to other cellular phenomena are emphasized. Primary focus with respect to the antigens evoking the in vitro and in vivo responses will be on the major histocompatibility complex; studies will be performed in both mouse and man. In vitro protocols involve careful evaluation of those factors in terms of antigen presentation that control the balance between help and suppression during the generation of the cytotoxic, or rejection, response. Preparation of reagents that help to define, and perhaps manipulate, different T lymphocyte subpopulations play an important role in the overall study. The overall experimental plan is aimed at obtaining information that will allow successful organ transplantation of not only endocrine organs such as the thyroid but hopefully other tissues as well.